To improve the printability of paper, the paper may be coated with a coating formulation containing mineral pigment and binder components. Over the years, application and levelling of the coat have been carried out using a variety of apparatuses. Higher web speeds and increased demands on process efficiency and paper quality in combination constitute the stimulus driving the development of applicator equipment.
Initially, paper coating with a pigment-containing formulation was performed using coaters of the gate roll type, in which the coating mix was first metered with the help of furnish rolls to a set of transfer rolls, and there-from further to the moving web of paper. However, the function of such a coater is impaired at web speeds exceeding 400 m/min. The nips of the rolls start to throw out splashes of the coating mix, and the coating process lacks the stability required to achieve an acceptable coat quality. Furthermore, well-behaved control of coat weight is difficult to achieve when using the above-described technique.
Particularly for surface sizing, sizing presses have been used in which the downward running web is passed through a coat mix pond sealed by the rolls. Herein, a problem arises from the strong increase of moisture content in the web and difficult controllability of the correct amount of applied size.
In the kiss-coating technique, the coating mix is metered directly in a nip from the casting roll to the surface of the paper web. In the early days, and in paperboard coating even today, excess coat is doctored away with the help of an air knife. At web speeds above 500 m/min, however, the impact force of the air flow from the slot orifice of the air knife is insufficient for effective doctoring of the coat layer applied to the web surface.
An essential increase in coating speed was facilitated by the adoption of the doctor blade levelling technique for controlling the final coat weight. In the first generation of blade coaters, the web was arranged to run from above downward, and the coating mix was pumped into a pond formed in the recess between the backing roll and the blade. In fact, the same technique is still being used in two-sided coating.
The actual break-through of the blade coating technique occurred along with the adoption of the transfer coating method. Herein, the coat is applied directly to the web surface in the nip between a transfer roll and a backing roll. Excess coat is removed by means of a doctor blade extending over the entire web width. This kind of coating technique makes it possible to increase the web speed to about 1300 m/min. At web speeds above this, splashing of the coat at the nip and the air film which is entrained in the nip along with the moving web, thereby causing skip marks on the coated web, make the use of this method extremely complex if not impossible. The higher the web speed, the fewer degrees of freedom will be available in the selection of coat mix components. Herein, the coating mix formulations must be selected under the constraints of web runnability, sometimes even compromising the quality of the end product.
Due to the poor runnability of transfer coaters, a short-dwell doctor blade coater was developed to provide an alternative technique for applying light coats to thin-caliper paper grades. In this type of coater, the web is guided past a slot orifice box which is formed by a short-dwell application chamber and the doctor blade and is adapted to operate against a backing roll. This method has been extremely popular in the art and facilitated effective on-machine coating. Also in this method, the maximum practicable web speed has turned out to be the limiting factor for further development. At web speeds above 1300 m/min, striping will appear at coat weights higher than 9 g/m.sup.2 due to turbulence in the applicator flow chamber. In addition, an essential impairment of the cross-machine coat profile occurs with higher coat weights.
Improvements in the design of film transfer-type coaters typically used in surface sizing of paper have extended the use of these coaters to the application of pigment coats, too. Herein, the coating mix is first metered by means of an apparatus similar to a short-dwell coater onto a transfer roll, wherefrom the coat film is further transferred in a nip of two rolls to the surface of the paper web. This novel technique was initially introduced to surface sizing and later also to the application of a pigment coat at unconventionally high web speeds. However, problems are encountered in the form of coat mist and splashing occurring at the splitting point of the coat film when the web exits the film transfer nip. When applied at high web speeds, coats heavier than 10 g/m.sup.2 suffer from an orange-peel texture and other low-quality surface properties incapable of fulfilling all the specifications that are set on a finished end product.
The coat splashing and web skipping problems occurring on the application roll have generally been overcome by means of the nozzle application technique, which gives a wider latitude in the direction of higher web speeds. Additionally, better capabilities of applying heavy coat weights have been attained through more effective water drainage offered by the longer dwell time. Moreover, the coat forms a layer of higher solids content close to the base sheet surface that provides support to the doctor blade, whereby blade stability is improved and cross-machine profiles of improved evenness are attained.
When the nozzle-application step based on a doctor blade and a subsequent levelling step based on a scraper element are performed against the same backing element, a runnability complication in the form of creases and/or bags in the web generally occurs. This problem can be eliminated by implementing the application and levelling steps against separate backing elements. Due to the resultant increase of dwell time and paper moisture content, some difficulties will be encountered in the runnability of lightweight and high-moisture-absorbance paper grades.
The striping problem of short-dwell coaters has been alleviated with the dam blade construction known from the film transfer method of coating. However, all the above-described application methods are hampered by the mechanical contact and load imposed on the web by the coater. Particularly in blade coaters, paper production will easily be disrupted by defects in the base sheet. Paper mills have a strong drive to improve the efficiency of coater lines. Obviously, valuable production time will be lost due to web breaks. In conventional application techniques, the time to regain an acceptable quality after a web break takes an unduly long time.
For wet-on-wet coating, a blade coater is not necessarily the best possible alternative. In this coating method, to the same side of the web are applied at least two coat layers so that onto the first coat, while still moist, is directly applied the next coat layer without intermediate drying. Particularly in the application of a precoat, web defects like striping and unevenness are extremely detrimental. Therefore, a blade coater requires continuous control to keep the coat weight at its set value. Hence, a facility for measuring the precoat weight is mandatory in order to maintain controlled coat application. Such a coat weight measurement system operating between the successive application steps of coat layers is expensive and sometimes even impossible to arrange. Therefore, stable operation is required from wet-on-wet coaters so that the application and levelling of subsequent coat layers can be carried out without spoiling the already applied, still moist coat layers.
Attempts have been made to improve runnability in paper machines and coating stations with the help of supported web threading. Herein, an extremely smooth surface is required from the support wires or belts used in coaters. Furthermore, even the smallest irregularities of backing surfaces will cause coat marking not only particularly in blade coaters, but also in transfer coaters.
At higher web speeds, the rate of successfully performed flying splicing on the unwinder of off-machine coaters falls significantly. Splicing apparatuses required herein become expensive, and nevertheless problems will occur in exact timing of splicing. Therefore, future development of coaters must aim to provide an on-machine coater embodiment in which such problems associated with splicing and roll change cannot disturb the finishing treatment.
A blade performing the doctoring of the coat applied to the web tends to accumulate aggregations of dirt under the blade edge that cause striping of the coat. Due to such coating defects, large amounts of finished paper turn into scrap.
The rheological properties of the coating mix may cause web runnability problems due to the extremely strong fields of high shear rate acting on the coat mix in the blade tip region. Accordingly, the selection of possible coating mix formulations is often curtailed by the rheological constraints associated with the blade geometry.
In order to overcome the above-described drawbacks, paper coating should preferably be carried out using a noncontacting method. Through the use of a noncontacting method for coating the web, defects of the base sheet are prevented from disturbing the finishing treatment. Complemented with a web threading system which is fully supported by wires and belts, it is possible to achieve a break-free, even a fully automated coating process. Herein, paper web defects can be identified by means of defect detectors and removed during intermediate winding in order to prevent them from interfering with further processing. Development of equipment for higher web speeds is no more hampered by load imposed on the web. The opacifying power of the applied coat becomes so good that the air knives, which today are the major factor limiting the maximum speed of paperboard coaters, can be replaced by the novel technique. Thus, the efficiency of coating lines and production throughput of coaters can be elevated to a remarkably high level.
In other prior art non-contacting coating methods such as that disclosed in PCT/US91/03830, the coating mix is fed into the nozzle via a separate duct, and atomization of coating mix is performed with the help of compressed air passed to the nozzle. However, tests to be described in greater detail later in the text have shown that insufficient atomization results from the use of a nozzle based on blast-diffusion by compressed air. Moreover, such a strong airflow causes excessive evaporative drying of the coating mix droplets before they can impinge on the sheet surface. Droplets of excessive size in the coat mist make the finished surface pitted and unsmoothly coated, which is manifested in the coat profile as craters and mounds.
U.S Pat. No. 4,944,960, discusses applicator apparatuses in which the coating mix aerosol is formed in a separate chamber or apparatus using a gas-liquid nozzle or ultrasonic diffusor nozzle. The coat aerosol is passed into an applicator nozzle, wherein the aerosol is directed by means of separate gas injection to impinge on the sheet surface. The portion of the coating mix aerosol not adhering to the web is returned by suction back into the coating mix circulation. In such an apparatus, the coating mix droplets undergo evaporation before reaching the sheet surface, whereby their adherence to the sheet is impaired. Subsequently, when the paper is used in a printing shop, a large amount of dirt will build on the printing machine rolls and the coat will release dust in the trimming and folding equipment.
In the apparatus described in WO 94/116, the coat is applied using the above-described methods and then levelled using a doctor unit. This method represents a kind of direct application with the exception of its conventional doctor blade technique, the shortcomings of which were described above.
Noncontacting coater equipment are well-known and frequently used apparatuses in the art of painting and coating systems technology. High-pressure spraying equipment with suitable nozzles are commercially available for painting. However, the use of high-pressure spraying for applying coating mix to a moving web of paper or paperboard in the fashion described in detail later is a novel application of the noncontacting application technique.
In order to make it possible to spray a coating mix or material onto a surface to be coated, the fluid material must be dispersed into small droplets. This step is called atomization. The basic idea of atomization covers a variety of different uses ranging from painting to varied combustion installations, engines and apparatuses for mass and heat transfer such as gas scrubbers and evaporation towers. As a general term, atomization refers to conversion of fluid material into droplet form (that is, particles of round or similar form). The type of the spray is categorized according to the cross-sectional shape of the spray jet. Normally, a hollow or solid conical or fanned spray is used. Spray coverage is defined as the width of the spray pattern at a certain distance from the nozzle tip. The spray angle is the opening angle of the spray cone emitted by the nozzle.
Atomization nozzles fall into four different classes:
1) High-pressure airless nozzles (pressure atomizers) PA1 2) Atomizers based on rotary centrifugal atomization (rotary atomizers) PA1 3) Air-assist and air-blast nozzles (twin-fluid atomizers) PA1 4) Other methods. PA1 for the nozzle types used in the test, the viscosity of the coating mix was too high to permit sufficient atomization of the coating mix to apply a smooth coat; PA1 coating mix droplets did not gain sufficient kinetic energy to adhere and spread sufficiently on the sheet surface; and PA1 pressure levels used in the fluid atomizing nozzles were insufficient for the atomization of the coating mix.
High-pressure atomizers are characterized in that therein atomization occurs driven alone by the internal pressure of the fluid being atomized. No atomizing air is used. In practical tests, airless atomizing nozzles have been found superior to air-blast nozzles.
In pilot-scale tests of the present invention, the spraying technique was first adapted to the application step of the coating mix. Levelling of the applied coat was performed using conventional doctor blade techniques. However, this combination did not offer any benefit over prior-art nozzle application methods.
The following shortcomings were found in this method:
Coating mix used in the atomization application method must have a sufficiently high kinetic energy to drive the coat droplets formed at the nozzle web against the sheet surface to flatten and adhere to the web surface. At higher web speeds, the droplets must also be capable of penetrating the barrier formed by the air film travelling along with the moving sheet surface. These requirements cannot be fulfilled by means of an air-blast atomizing nozzle. This is because the blasting air flow causes strong evaporation of the coat droplets, whereby the deposition and spreading of the coating mix droplets on the sheet surface is worsened. Hence, the achievable coat quality remains unsatisfactory.